Plastic is not soluable in water and doesn't really break down to most organic acids, so most life just can't really interact with it. I think the main reason micro plastics interfere with our biological processes is because it just jams up our cells, it doesn't really do much else(although I'm not sure about this). There's also nitrogen everywhere, but most animals can't get much out of it unless it's already been bound to something else by bacteria.